Plasma‐Assisted Defect Engineering on p‐n Heterojunction for High‐Efficiency Electrochemical Ammonia Synthesis

Abstract A defect‐rich 2D p‐n heterojunction, Co x Ni3‐ x (HITP)2/BNSs‐P (HITP: 2,3,6,7,10,11‐hexaiminotriphenylene), is constructed using a semiconductive metal–organic framework (MOF) and boron nanosheets (BNSs) by in situ solution plasma modification. The heterojunction is an effective catalyst for the electrocatalytic nitrogen reduction reaction (eNRR) under ambient conditions. Interface engineering and plasma‐assisted defects on the p‐n CoxNi3‐x(HITP)2/BNSs‐P heterojunction led to the formation of both Co‐N3 and B…O dual‐active sites. As a result, Co x Ni3‐x(HITP)2/BNSs‐P has a high NH3 yield of 128.26 ± 2.27 µg h−1 mgcat. −1 and a Faradaic efficiency of 52.92 ± 1.83% in 0.1 m HCl solution. The catalytic mechanism for the eNRR is also studied by in situ FTIR spectra and DFT calculations. A Co x Ni3‐ x (HITP)2/BNSs‐P‐based Zn‐N2 battery achieved an unprecedented power output with a peak power density of 5.40 mW cm−2 and an energy density of 240 mA h gzn −1 in 0.1 m HCl. This study establishes an efficient strategy for the rational design, using defect and interfacial engineering, of advanced eNRR catalysts for ammonia synthesis under ambient conditions.


Plasma-Assisted Defect Engineering on p-n Heterojunction for High-Efficiency Electrochemical Ammonia Synthesis
Jiameng Liu, Linghao He, Shuangrun Zhao, Sizhuan Li, Lijun Hu, Jia-Yue Tian, Junwei Ding, Zhihong Zhang*, and Miao Du* mg of sodium borohydride powder was heated from room temperature to 490 °C at a rate of 10 °C min -1 for 2 h, and continuously heated to 550 °C at a rate of 5 °C min -1 for 30 min to form stable intermediates. Then, the intermediate was heated to 600 °C at a rate of 5 °C min -1 for 30 min to obtain BNSs. Notably, the whole process was taken under the protection of Ar.
After cooling down, the product was washed with water to remove unreacted precursors and dried at 60 °C under vacuum.

Plasma-modification procedure
CoxNi3-x(HITP)2/BNSs-P was obtained according to the following procedure. Typically, 2 mg CoxNi3-x(HITP)2 was dispersed in 1.9 mL water in a glass vial, following by sonicated for 10 min. Then, 3 mg BNSs were added into the CoxNi3-x(HITP)2 suspension and sonicated for 30 min (denoted as CoxNi3-x(HITP)2/BNSs). Subsequently, the glass vial was placed into the plasma chamber and irradiated with a continuous wave for 5 min at a plasma input power of 200 W under a pressure of 0.1 Pa. Afterward, 100 μL of 5 wt% Nafion was added to the obtained solution and sonicated for 30 min to form a homogeneous ink. The prepared CoxNi3x(HITP)2/BNSs-P catalyst was stored for further electrochemical measurements. In addition, the CoxNi3-x(HITP)2 suspension (40 μL, 2 mg mL -1 ) was coated on the carbon paper, and dried in air. After that, the BNSs suspension (40 μL, 2 mg mL -1 ) was dropped on the CoxNi3x(HITP)2-modified carbon paper (represented by CoxNi3-x(HITP)2-BNSs). In addition, the modification step of the carbon paper was upside down. The achieved electrode was then denoted as BNSs-CoxNi3-x(HITP)2.

Electrocatalytic nitrogen reduction reaction (eNRR) measurements
Electrochemical measurements were performed on an H-type-cell equipped with a CHI 760E workstation (CH Instruments, Inc., Shanghai, China), which was separated by the Nafion 117 membrane. The CoxNi3-x(HITP)2/BNSs-P electrode, Ag/AgCl (Saturated KCl) electrode and graphite rod work as the working, reference, and counter electrodes, respectively, using 0.1 M HCl as the electrolyte. The prepared catalyst (2 mg) was dispersed in 0.95 mL of water, followed by ultrasonication for 30 min to form homogeneous dispersion. Then, the suspension was mixed with 0.05 mL of 5% Nafion solution. Subsequently, 80 μL of the mixture was dipped onto carbon paper (1 × 1 cm 2 ) with a loading mass of 0.16 mg cm -2 , and dried at room temperature. Before each eNRR process, the used electrolyte was pretreated by a refrigerated pumping process for three cycles to ensure removal of air. After that, the self-built H-cell was bubbled continuously with N2 gas for at least 30 min. Subsequently, the gas was maintained throughout electrochemical reaction. Cyclic voltammetry (CV) tests were carried out at a scan rate of 50 mV s -1 at the applied potential of -0.6 V and -0.2 V vs. RHE in N2-saturated electrolyte. Potentiostatic measurements were taken at a series of applied potentials including -0.2, -0.3, -0.4, -0.5, and -0.6 V vs. RHE for 6000 s under constant room temperature. The potentials were converted to RHE scale via calibration with the following equation: E (vs. RHE) = E (vs. Ag / AgCl) + 0.22 V.

Statistical Analysis
All measurements were made in triplicate and presented as mean±standard deviation. Statistical significance between two groups was analyzed by the Student's t-test. In addition, there was no significant difference between the two electrodes in parallel test for 3 times (P=0.909>0.005), indicating the reliability of the data.

Detection of ammonia
Indophenol blue method was used to estimate the concentration of ammonia in the electrolyte containing 0.1 M HCl after the electrochemical measurement for 6000 s. The color reagent system was prepared: solution A, 1 M NaOH solution containing 5 wt% salicylic acid and 5 wt% sodium citrates; solution B, 0.05 M NaClO; solution C, 1 wt% C5FeN6Na2O (sodium nitroferricyanide) aqueous solution. The concentration-absorbance curves were established using a series of standard ammonia solutions. First, 2 mL of post-tested electrolyte solution was removed from electrochemical system, followed by the sequential addition of 1 mL of solution A, and 0.2 mL of solution C. After keeping for 2 h at room temperature, the absorption spectrum was measured on an ultraviolet-visible spectrophotometer. The absorbance intensity at 655 nm was utilized to estimate the yield of ammonia based on the standard curve.

Detection of hydrazine
Concentration of hydrazine was spectrophotometrically determined using Watt and Chrisp method. A mixture of para-(dimethylamino) benzaldehyde (5.99 g), HCl (concentrated, 30 mL), and ethanol (300 mL) was used as a color reagent. 3 mL of solution was taken out from the electrochemical reaction vessel and the above color reagent was added. The mixture was kept stirring for 10 min at room temperature. The amount of hydrazine formed during the electrolysis was determined based on the absorbance intensity at 455 nm. The concentrationabsorbance curve was established by standard hydrazine solution, which contains the same concentration of HCl as used in each electrolysis experiment.

Isotope labeling experiment
Before the test, the labeled 15 N2 (99 % 15 N) as the feed gas was pre-purified through flowing into NaOH solution, KMnO4 solution, and then passing through a volumetric flask with twothirds of Na2SO4 solution to remove any N contamination, and then the gas flowed out was collected. In the eNRR measurement, Ar gas was purged to the cathodic cell to remove impurity gas and purge for 30 min with the gas to be tested. After testing in 0.1 M HCl electrolyte for 6000 s, 20 mL of the electrolyte was taken out and then concentrated to 2 mL by heating via reduced pressure distillation. Afterward, 0.9 mL of the resulting solution was mixed with 0.10 mL DMSO-d 6 for the 1 H NMR measurements.

Calculation of ammonia yield rate and Faradaic efficiency
The Faradaic efficiency (FE) for the eNRR was defined as the amount of electric charge used for NH3 synthesis divided by the total charge passed through the electrodes during electrolysis.
= 0.3011 + 0.0322 Among them, Y is the value obtained by UV spectra testing the solution after color development, and r(NH3) is the rate of NH3 production.
Assuming three electrons were required to produce one NH3 molecule, the FE was calculated as: is the measured concentration of NH4 + , V is the volume of HCl electrolyte in the cathode chamber, i is the instantaneous current measured by chronoamperometry.

Characterizations
Field emission scanning electron microscopy (FE-SEM) images were obtained with a JSM-6700F (JEOL) operating at 5 kV in LABE mode. TEM images were obtained with a Tecnai Synchrotron X-ray absorption spectroscopy (XAS) data were processed and analyzed using the Demeter software package. A linear function was subtracted from the pre-edge region, then the edge jump was normalized using Athena software. The χ(k) data were isolated by subtracting a smooth, polynomial approximating the absorption background of an isolated atom. The k weighted χ(k) data were Fourier transformed after applying window function.
The global amplitudes were obtained by nonlinear fitting, with least-squares refinement, of the EXAFS equation to the Fourier-transformed data in R-space, using Artemis software.